Field of the Invention
The present invention relates to the field of measuring the geometry of single fiber optic connectors and ferrules by means of interferometric microscopes. More specifically, the invention relates to interferometric measurement of fiber core concentricity, angular misalignment between fiber and ferrule axes and end face polish angle.
Description of the Related Art
Manufacturers of fiber optic connectors seek ways to produce the connectors with low transmission loss and low back reflection. End faces of fiber optic connectors must satisfy certain criteria for effective fiber mating as required by the industry standards. They must be clean and their surface geometry must provide for good physical contact, low signal loss and back reflection.
As set by the industry standards, the performance of single fiber optic connection is determined by fiber core concentricity and angular alignment of the connectors being mated. FIG. 1A-1D demonstrate the parameters influencing the performance by two examples of low-loss and high-loss connections.
Good cable performance is presented on FIG. 1A and FIG. 1B. Fiber cores 3 and 3′ are concentric and well aligned with claddings 1 and 1′ in both mating connectors. FIG. 1A illustrates that optical signal propagates well between the connecting cables with no loss as shown by the horizontal arrows. FIG. 1B provides a cross-sectional view of a low-loss fiber optic connector 20 in which the cladding 1 is correctly aligned with the fiber bore 6. The fiber with the core 3 satisfies concentricity requirement relative to the connector key 8 with X and Y axes 2 and 2′.
FIG. 1C demonstrates how signal is extinguished at the juncture because the fiber with the core 3 is not concentric and misaligned in the cladding 1. FIG. 1D demonstrates a cross-sectional view of the fiber optic connector 20′ with low performance. The cladding 1 is misaligned in the fiber bore 6. The fiber with core 3 does not satisfy the concentricity requirement relative to the connector key 8 with X and Y axes 2 and 2′.
A method for measuring angular misalignment between the fiber and ferrule axes for fiber optic connectors with fiber installed is described in IEC 61-300-3-26 standard. According to the abovementioned method, the ferrule is placed in a V-groove and rotated. Displacement of the light beam that shines into the fiber core is detected by a microscope and a video camera for several angular positions of the ferrule.
The angular misalignment is measured as the deviation in the far field pattern coming from the fiber core (see FIG. 2). The reference number 25 designates zirconia ferrule with axis 7. The reference number 3 designates fiber core with Z axis 9. The movement of the light beam forms a circle 12 on the CCD target 11. The angle of misalignment 13 between the ferrule axis 7 and fiber axis 9 is calculated.
The disadvantage of the angular misalignment measurement method is that the rotation axis can deviate from the initial position that can influence the accuracy of results. This is illustrated on FIG. 2 by a deviation angle 10. The rotation axis in its initial position coincides with the ferrule axis 7 and then shifts to the position 7′.
There are also other known methods for measuring the angular misalignment of fiber optic connector components. For example, in U.S. Pat. No. 6,918,269 B2 Hua-Kuan Wang, P., (2005) it is suggested to rotate an optical fiber or a device about three axes so that rotation of the fiber does not cause a translation of the end of the fiber. The rotation is made until maximum signal is achieved with another optical device. Additional equipment is required for such rotation.
For measuring the angular misalignment between the fiber bore and the ferrule axes, the present invention suggests inserting a reference fiber into the fiber bore. Then this inserted fiber and the ferrule are scanned from side by an interferometric microscope and their 3D positions are determined. It is possible to calculate relative angle between the ferrule and fiber bore axes. There is no need in rotation in such method and it is simpler and requires no additional equipment beside an interferometer used for conventional end face testing.
It should be understood that the rotation of the ferrule can be applied in the present method to improve the accuracy of calculation. The deviation of the rotation axis does not affect the accuracy of results because the present method allows determining 3D position of the axis for each angular position.
A method for measuring the fiber core concentricity of ferrules with fiber bores and of ferrules with fiber installed is described in the standard IEC 61-300-3-25. According to the abovementioned method, the ferrule is placed in a V-groove and rotated. Displacement of the light beam that shines into the fiber core is detected by a microscope and a video camera for several angular positions of the ferrule.
The fiber core concentricity is calculated as diameter of a circle created by the movement of fiber core center (see FIG. 3). The reference number 1 designates a cladding with X and Y axes 2 and 2′ respectively. The cladding contains a fiber with a core 3 with X and Y axes 4 and 4′ respectively.
When the ferrule is rotated in the V-groove, the fiber core trajectory forms a circle 5. The fiber core concentricity is defined by the circle diameter 5′.
There are also other known methods for measuring the fiber core concentricity. For example, in U.S. Pat. No. 6,421,118 B1 Shaar, C., (2002) it is suggested to image fiber end face on a detector and measure the location of the center of the core relative to the image of the center of the cladding for three angular positions. The object is being rotated about the axis of rotation to take its three angular positions. The concentricity is calculated using the three measured figures.
U.S. Pat. No. 5,367,372 A DiVita P., (1994) suggests measuring geometric characteristics of nominally cylindrical guiding structures. Both eccentricity of the internal element and non-circularity of the external surface are measured by detecting a curve during rotation similarly to the method described in the standard IEC 61-300-3-25.
The disadvantage of both the method described in IEC 61-300-3-25 and the methods presented in the above stated patents is that the rotation axis can deviate from its initial position and influence the accuracy of results.
The present invention suggests measuring the fiber core concentricity and the ferrule circularity of single fiber connectors by simultaneous or subsequent scanning of the zirconia ferrule side surface and the connector or ferrule end face. The ferrule core center location and location of the end point on the ferrule is determined for several angular positions. Although the connector is being rotated, the deviation of the rotation axis from its initial position has no influence on the accuracy of measurement. The reason is that a relative position of the end point on the ferrule and the fiber core center is always determined.
The measurement method introduced in the present invention also allows increasing precision of measuring ferrule end face angle parameter. In IEC 61300-3-47 this parameter is defined as an angle between the plane perpendicular to the ferrule axis and a line tangent to the polished end face surface at the fiber center (see FIG. 4). The reference number 2 represents a zirconia ferrule with the axis 7. The plane 21 is perpendicular to the ferrule axis 7. The line 22 is tangent to the end face surface at the fiber center, and 23 is the end face polish angle.
The interferometric scanning of the ferrule side surface introduced in the present invention allows determining a precise 3D position of the ferrule axis. Thus, it allows to measure the end face polished angle with increased precision.
The present invention employs the same technique of measuring fiber optic connectors from side as described in related patent applications by the same inventor (see Towfiq, F., (2015) U.S. patent application Ser. No. 14/697,784 and Towfiq, F., (2015) U.S. patent application Ser. No. 14/744,314).